1. Field of the Invention
This invention relates to an energy subtraction processing method and apparatus, wherein energy subtraction processing is performed on a plurality of image signals representing radiation images of an object. This invention also relates to a recording medium, on which a program for causing a computer to execute the energy subtraction processing method has been recorded and from which the computer is capable of reading the program.
2. Description of the Related Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into a digital image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
Also, techniques for performing energy subtraction processing on radiation images have heretofore been known. (The energy subtraction processing techniques are disclosed in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037, 5,485,371.) With the energy subtraction processing techniques, an object is exposed to several kinds of radiation having different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object is changed after the radiation has been irradiated onto one of a plurality of radiation detecting means (e.g., the stimulable phosphor sheets described above), after which the radiation impinges upon the second radiation detecting means. In this manner, a plurality of radiation images, in which different images of a specific structure of the object are embedded, are obtained. Thereafter, image signal components of the image signals representing the plurality of the radiation images, which image signal components represent corresponding pixels in the radiation images, are multiplied by appropriate weight factors, and the thus weighted image signal components are subjected to a subtraction process. From the subtraction process, a difference signal, which represents only the image of the specific structure of the object, is obtained. By the utilization of the thus obtained difference signal, a visible radiation image, which represents only the specific structure of the object, is capable of being reproduced.
In the aforesaid radiation image recording and reproducing systems utilizing the stimulable phosphor sheets, the radiation image having been stored on the stimulable phosphor sheet is read out directly as an electric image signal. Therefore, with the radiation image recording and reproducing systems, the energy subtraction processing is capable of being performed easily. In cases where the energy subtraction processing is to be carried out by using the stimulable phosphor sheets, radiation images may be stored on, for, example, two stimulable phosphor sheets such that the parts of the radiation images corresponding to a specific structure may be different in the two radiation images. For such purposes, a two-shot energy subtraction processing technique may be employed wherein the operation for recording a radiation image is performed twice with two kinds of radiation having different energy distributions. Alternatively, a one-shot energy subtraction processing technique may be employed wherein, for example, two stimulable phosphor sheets placed one upon the other (they may be in contact with each other or spaced away from each other) are simultaneously exposed to radiation, which carries image information of an object, such that the two stimulable phosphor sheets are exposed to radiation having different energy distributions. In cases where the two-shot energy subtraction processing technique is employed, and radiation images are to be recorded on an increased number of the stimulable phosphor sheets, the same number of image recording operations as that of the stimulable phosphor sheets are performed. For example, in cases where the two-shot energy subtraction processing technique is employed, and radiation images are to be recorded on three stimulable phosphor sheets, three image recording operations are performed. Therefore, in this specification, the energy subtraction processing technique, in which multiple image recording operations are performed, is referred to as the multi-shot energy subtraction processing technique (including the two-shot energy subtraction processing technique).
With the energy subtraction processing techniques utilizing the stimulable phosphor sheets, for example, in cases where the object is a human body, two radiation images of the human body may be formed on the stimulable phosphor sheets with two kinds of radiation having different energy distributions (i.e., radiation having a high energy level and radiation having a low energy level), and two image signals representing the two radiation images may be obtained. Also, the two image signals may be weighted with appropriate weight factors, and the difference signal may be obtained by performing a subtraction process on the weighted image signals. In this manner, a radiation image, in which only the pattern of a soft tissue of the human body is illustrated, and a radiation image, in which only the pattern of a bone of the human body is illustrated, are capable of being obtained. The operation processing is performed in the manner described below. Specifically, the first stimulable phosphor sheet, upon which the radiation having a low energy level impinges, may be represented by IP1, and the second stimulable phosphor sheet, upon which the radiation having a high energy level impinges, may be represented by IP2. Also, a logarithmic value of a radiation dose (i.e., a logarithmic radiation dose), which the first stimulable phosphor sheet IP1 receives, may be represented by L, and the logarithmic value of the radiation dose (i.e., the logarithmic radiation dose), which the second stimulable phosphor sheet IP2 receives, may be represented by H. In such cases, L and H may be represented respectively by Formula (1) and Formula (2) shown below.
L=xe2x88x92{overscore (xcexcS+L )}tSxe2x88x92{overscore (xcexcB+L )}tB+L0xe2x80x83xe2x80x83(1)
H=xe2x88x92{overscore (xcexcS+L )}xe2x80x2tSxe2x88x92{overscore (xcexcB+L )}xe2x80x2tB+H0xe2x80x83xe2x80x83(2)
wherein
{overscore (xcexcB+L )} represents the mean attenuation coefficient of the bone with respect to the first stimulable phosphor sheet IP1,
{overscore (xcexcS+L )} represents the mean attenuation coefficient of the soft tissue with respect to the first stimulable phosphor sheet IP1,
{overscore (xcexcB+L )}xe2x80x2 represents the mean attenuation coefficient of the bone with respect to the second stimulable phosphor sheet IP2,
{overscore (xcexcS+L )}xe2x80x2 represents the mean attenuation coefficient of the soft tissue with respect to the second stimulable phosphor sheet IP2,
tS represents the thickness of the bone,
tB represents the thickness of the soft tissue, and
each of L0 and H0 represents the fixed number depending upon the radiation source.
As the logarithmic radiation doses L and H, the image signals having been obtained from the stimulable phosphor sheets IP1 and IP2 may be employed respectively.
A substance has a radiation attenuation coefficient depending upon radiation energy. Also, in cases where the radiation irradiated to the object is not monochromatic and is distributed over a certain energy range, the energy distribution of the detected radiation (e.g., the radiation impinging upon the stimulable phosphor sheet) changes depending upon the thickness of a substance contained in the object (in cases where the object is the human body, a bone or a soft tissue). Such a phenomenon is referred to as the beam hardening. Therefore, the radiation attenuation coefficient of the substance is weighted with the energy distribution of the detected radiation and averaged. The thus obtained value is defined as the mean attenuation coefficient. Accordingly, the mean attenuation coefficient varies for different thicknesses of the substance.
In cases where L represented by Formula (1) shown above is multiplied by the factor of {overscore (xcexcB+L )}xe2x80x2, and H represented by Formula (2) shown above is multiplied by the factor of {overscore (xcexcB+L )}, the difference between {overscore (xcexcB+L )}H and {overscore (xcexcB+L )}xe2x80x2L maybe represented by Formula (3) shown below.
{overscore (xcexcB+L )}Hxe2x88x92{overscore (xcexcB+L )}xe2x80x2L=({overscore (xcexcS+L )}xe2x80x2{overscore (xcexcB+L )}xe2x80x2xe2x88x92{overscore (xcexcS+L )}xe2x80x2{overscore (xcexcB+L )})tS+({overscore (xcexcB+L )}H0xe2x88x92{overscore (xcexcB+L )}xe2x80x2L0)xe2x80x83xe2x80x83(3)
In this manner, a difference signal, which represents a soft tissue image illustrating only the soft tissue and is free from the thickness of the bone, is capable of being obtained.
Also, in cases where L represented by Formula (1) shown above is multiplied by the factor of {overscore (xcexcS+L )}xe2x80x2, and H represented by Formula (2) shown above is multiplied by the factor of {overscore (xcexcS+L )}, the difference between {overscore (xcexcS+L )}H and {overscore (xcexcS+L )}xe2x80x2L may be represented by Formula (4) shown below.
{overscore (xcexcS+L )}Hxe2x88x92{overscore (xcexcS+L )}xe2x80x2L=({overscore (xcexcS+L )}xe2x80x2{overscore (xcexcB+L )}xe2x88x92{overscore (xcexcS+L )}{overscore (xcexcB+L )}xe2x80x2)tB+({overscore (xcexcS+L )}xe2x80x2H0xe2x88x92{overscore (xcexcS+L )}xe2x80x2L0)xe2x80x83xe2x80x83(4)
In this manner, a difference signal, which represents a bone image illustrating only the bone and is free from the thickness of the soft tissue, is capable of being obtained.
In each of Formula (3) and Formula (4) shown above, the mean attenuation coefficient, by which L is multiplied, and the mean attenuation coefficient, by which H is multiplied, act as the weight factors.
The mean attenuation coefficients, which are employed as the weight factors in each of Formula (3) and Formula (4) shown above, are determined by, for example, being presumed from the image signal having been obtained from the stimulable phosphor sheet, upon which the radiation having a low energy level impinged. Therefore, in cases where the energy subtraction processing is performed, an identical value of the mean attenuation coefficient is employed as the weight factor with respect to all of the pixels in each radiation image. However, the thickness of the substance contained in the object ((in cases where the object is the human body, the bone or the soft tissue) varies for different sites in the object. Also, as described above, the mean attenuation coefficient varies for different thicknesses of the substance contained in the object. Therefore, for example, in cases where the object is the human body, the thickness of the soft tissue or the thickness of the bone is not uniform in accordance with the site in the object. Accordingly, in cases where an identical value of the mean attenuation coefficient is employed as the weight factor with respect to all of the pixels in each radiation image, the problems occur in that an unnecessary structure pattern cannot be removed perfectly. As a result, the bone pattern remains unremoved in the soft tissue image, and the soft tissue pattern remains unremoved in the bone image.
The primary object of the present invention is to provide an energy subtraction processing method, wherein only a specific structure pattern contained in an object image is capable of being extracted accurately.
Another object of the present invention is to provide an apparatus for carrying out the energy subtraction processing method.
The specific object of the present invention is to provide a recording medium, on which a program for causing a computer to execute the energy subtraction processing method has been recorded and from which the computer is capable of reading the program.
As will be described later, in cases where a difference between logarithmic values of radiation doses at the time of formation of two radiation images is calculated, or a logarithmic value of a ratio between the radiation doses at the time of formation of two radiation images is calculated, a certain relationship is obtained between the difference between the logarithmic values of the radiation doses, or the logarithmic value of the ratio between the radiation doses, and the mean attenuation coefficient of a certain substance with respect to each of the two radiation images, which are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses. In cases where the relationship described above is determined previously, if the difference between the logarithmic values of the radiation doses or the logarithmic value of the ratio between the radiation doses is found, the mean attenuation coefficient of the certain substance with respect to each of the two radiation images, which are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses, will be capable of being calculated. A first energy subtraction processing method and a first energy subtraction processing apparatus in accordance with the present invention are based on such findings.
Specifically, the present invention provides a first energy subtraction processing method, comprising the steps of:
i) obtaining a plurality of image signals, each of which represents one of a plurality of radiation images of a single object and is made up of a series of image signal components, the plurality of the radiation images having been formed respectively with a plurality of kinds of radiation (such as X-rays or xcex3-rays) having different energy distributions and carrying image information of the object, different images of at least part of the object being embedded in the plurality of the radiation images,
ii) weighting each of the image signals with a predetermined weight factor, and
iii) performing a subtraction process on the image signal components of the weighted image signals, which image signal components represent corresponding pixels in the radiation images, a difference signal representing an image of a specific structure of the object being thereby obtained,
wherein the improvement comprises the step of:
setting the predetermined weight factor with respect to each of pixels in each of the radiation images and in accordance with a difference between logarithmic values of radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images, or in accordance with a logarithmic value of a ratio between the radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images.
In order for the image signals representing the radiation images to be obtained, the plurality of kinds of the radiation having different energy distributions and carrying the image information of the object may be caused to impinge upon radiation detecting means, such as stimulable phosphor sheets or semiconductor sensors, and the image signals in accordance with the received radiation doses may be detected by the radiation detecting means. In cases where the radiation detecting means is constituted of the semiconductor sensors, a signal outputted from each of the semiconductor sensors may be taken as the image signal. In cases where the radiation detecting means is constituted of the stimulable phosphor sheets, in the same manner as that in the radiation image recording and reproducing systems described above, each of the stimulable phosphor sheets may be exposed to the stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, the emitted light may be photoelectrically detected, and the image signal may thereby be obtained. The formation of the plurality of the radiation images may be performed with the one-shot energy subtraction processing technique or the multi-shot energy subtraction processing technique.
The predetermined weight factor corresponds to each of the mean attenuation coefficients in Formula (3) or Formula (4) shown above. With respect to one radiation image, the same number of the mean attenuation coefficients as that of the kinds of the specific structures contained in the object are set. For example, in cases where the specific structures are the soft tissue and the bone of the human body, one mean attenuation coefficient of the soft tissue and one mean attenuation coefficient of the bone are obtained with respect to one radiation image. Therefore, in cases where three radiation images are to be processed, and the specific structures are the soft tissue and the bone of the human body, six kinds (=3xc3x972) of the mean attenuation coefficients are obtained.
The term xe2x80x9cradiation dose at the time of formation of a radiation imagexe2x80x9d as used herein means the dose of radiation, which carries the image information of the object and impinges upon the radiation detecting means when the image of the object is formed on the radiation detecting means. The radiation dose is capable of being obtained by directly detecting the radiation impinging upon the radiation detecting means. However, it is not always possible to detect the radiation dose with respect to each of the pixels in the radiation image. However, when the dose of radiation impinging upon the radiation detecting means is large, the image signal obtained from the radiation detecting means takes a large signal value. Thus the signal value of the image signal has the correspondence relationship with the radiation dose. Therefore, the image signal, which has been obtained from the radiation detecting means (and has not yet been subjected to image processing), should preferably be regarded as the radiation dose and utilized for the setting of the mean attenuation coefficient.
In cases where the radiation doses with respect to two radiation images are represented by I1 and I2, the relationship represented by the formula ln(I1)xe2x88x92ln(I2)=ln(I1/I2) is obtained. Therefore, the xe2x80x9cdifference between the logarithmic values of the radiation doses with respect to the corresponding pixels in the radiation imagesxe2x80x9d and the xe2x80x9clogarithmic value of the ratio between the radiation doses with respect to the corresponding pixels in the radiation imagesxe2x80x9d take an identical value.
As for the relationship between the predetermined weight factor and the difference between the logarithmic values of the radiation doses with respect to the corresponding pixels in the radiation images, or the relationship between the predetermined weight factor and the logarithmic value of the ratio between the radiation doses with respect to the corresponding pixels in the radiation images, a table representing the relationship or a functional expression representing the relationship may be utilized. In cases where the table representing the relationship described above is utilized, reference may be made to the table, and the predetermined weight factor may thereby be set. In cases where the functional expression representing the relationship is utilized, operation processing may be performed with the functional expression, and the predetermined weight factor may thereby be set.
The relationship between the predetermined weight factor and the difference between the logarithmic values of the radiation doses describe above, or the relationship between the predetermined weight factor and the logarithmic value of the ratio between the radiation doses described above, varies for different image recording conditions employed in the image recording operation, such as the voltage of a radiation source, the kind of the radiation source, and the sensitivity of the radiation detecting means. Therefore, a plurality of tables or functions in accordance with various different image recording conditions should preferably be prepared previously, and a table or a function should preferably be selected in accordance with the image recording conditions. Also, the predetermined weight factor should preferably be set by the utilization of the thus selected table or the thus selected function.
The present invention also provides a first energy subtraction processing apparatus, comprising:
i) means for obtaining a plurality of image signals, each of which represents one of a plurality of radiation images of a single object and is made up of a series of image signal components, the plurality of the radiation images having been formed respectively with a plurality of kinds of radiation having different energy distributions and carrying image information of the object, different images of at least part of the object being embedded in the plurality of the radiation images, and
ii) means for weighting each of the image signals with a predetermined weight factor, and performing a subtraction process on the image signal components of the weighted image signals, which image signal components represent corresponding pixels in the radiation images, in order to obtain a difference signal representing an image of a specific structure of the object,
wherein the improvement comprises the provision of:
setting means for setting the predetermined weight factor with respect to each of pixels in each of the radiation images and in accordance with a difference between logarithmic values of radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images, or in accordance with a logarithmic value of a ratio between the radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images.
The first energy subtraction processing apparatus in accordance with the present invention should preferably be modified such that the apparatus further comprises storage means for storing information representing a table or a function, which represents a relationship between the predetermined weight factor and the difference between the logarithmic values of the radiation doses with respect to the corresponding pixels in the radiation images, or which represents a relationship between the predetermined weight factor and the logarithmic value of the ratio between the radiation doses with respect to the corresponding pixels in the radiation images, the relationship having been determined previously, and
the setting means makes reference to the table or the function having been stored in the storage means and sets the predetermined weight factor.
In such cases, the first energy subtraction processing apparatus in accordance with the present invention should more preferably be modified such that the storage means stores a plurality of tables or functions, which represent the relationships having been set in accordance with various different image recording conditions at the time of formation of radiation images, and
the setting means accepts selection of a table or a function in accordance with image recording conditions having been set at the time of the formation of the plurality of the radiation images, makes reference to the thus selected table or the thus selected function, and thereby sets the predetermined weight factor.
The present invention further provides a first recording medium, on which a program for causing a computer to execute the first energy subtraction processing method in accordance with the present invention has been recorded and from which the computer is capable of reading the program.
As described above, in cases where the difference between the logarithmic values of the radiation doses at the time of the formation of two radiation images is calculated, or the logarithmic value of the ratio between the radiation doses at the time of the formation of the two radiation images is calculated, a certain relationship is obtained between the difference between the logarithmic values of the radiation doses, or the logarithmic value of the ratio between the radiation doses, and the mean attenuation coefficient of a certain substance with respect to each of the two radiation images, which are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses. In cases where the relationship described above is determined previously, if the difference between the logarithmic values of the radiation doses or the logarithmic value of the ratio between the radiation doses is found, the mean attenuation coefficient of the certain substance with respect to each of the two radiation images, which are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses, will be capable of being calculated. Also, in cases where at least three radiation images are formed, the mean attenuation coefficient of a certain substance with respect to one of the at least three radiation images has a certain relationship with the difference between the logarithmic values of the radiation doses with respect to the other two radiation images, or with the logarithmic value of the ratio between the radiation doses with respect to the other two radiation images. Therefore, the mean attenuation coefficient of the certain substance with respect to the one radiation image is capable of being calculated in accordance with the difference between the logarithmic values of the radiation doses with respect to the other two radiation images, or in accordance with the logarithmic value of the ratio between the radiation doses with respect to the other two radiation images. Further, in the strict sense, the thus calculated mean attenuation coefficient deviates from a true value due to adverse effects of a noise and scattered radiation. A second energy subtraction processing method and a second energy subtraction processing apparatus in accordance with the present invention are based on such findings.
Specifically, the present invention still further provides a second energy subtraction processing method, comprising the steps of:
i) obtaining a plurality of image signals, each of which represents one of a plurality of, at least three, radiation images of a single object and is made up of a series of image signal components, the plurality of the radiation images having been formed respectively with a plurality of kinds of radiation (such as X-rays or xcex3-rays) having different energy distributions and carrying image information of the object, different images of at least part of the object being embedded in the plurality of the radiation images,
ii) weighting each of two representative image signals, which are representative of the plurality of the image signals, with a predetermined weight factor, and
iii) performing a subtraction process on the image signal components of the weighted image signals, which image signal components represent corresponding pixels in the two radiation images represented by the two representative image signals, a difference signal representing an image of a specific structure of the object being thereby obtained,
wherein the improvement comprises the steps of:
a) setting a mean attenuation coefficient with respect to each of all of the radiation images, with respect to each of pixels in each of the radiation images, and in accordance with a difference between logarithmic values of radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images, or in accordance with a logarithmic value of a ratio between the radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images, the setting of the mean attenuation coefficient being performed for each of combinations of two radiation images, which two radiation images are associated with a calculation of the difference between the logarithmic values of the radiation doses or a calculation of the logarithmic value of the ratio between the radiation doses and are selected from the plurality of the radiation images,
b) calculating a mean value of the mean attenuation coefficients, which have thus been set with respect to an identical radiation image among all of the radiation images and for all of the combinations of the two radiation images selected from the plurality of the radiation images, the mean value of the mean attenuation coefficients being calculated with respect to each of pixels in the identical radiation image, a plurality of mean values being obtained with respect to all of the radiation image, and
c) setting representative values of the mean values, which representative values correspond to the radiation images represented by the two representative image signals, as the predetermined weight factors for the two representative image signals.
In the second energy subtraction processing method in accordance with the present invention, as in the first energy subtraction processing method in accordance with the present invention, in order for the image signals representing the radiation images to be obtained, the plurality of kinds of the radiation having different energy distributions and carrying the image information of the object may be caused to impinge upon the radiation detecting means, such as stimulable phosphor sheets or semiconductor sensors, and the image signals in accordance with the received radiation doses may be detected by the radiation detecting means. In cases where the radiation detecting means is constituted of the semiconductor sensors, a signal outputted from each of the semiconductor sensors may be taken as the image signal. In cases where the radiation detecting means is constituted of the stimulable phosphor sheets, in the same manner as that in the radiation image recording and reproducing systems described above, each of the stimulable phosphor sheets may be exposed to the stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, the emitted light may be photoelectrically detected, and the image signal may thereby be obtained. The formation of the plurality of the radiation images may be performed with the one-shot energy subtraction processing technique or the multi-shot energy subtraction processing technique.
In the second energy subtraction processing method in accordance with the present invention, the energy subtraction processing is performed on the two representative image signals, which are representative of the plurality of the image signals representing the plurality of (at least three) radiation images. In cases where the image recording operation is performed by utilizing at least three radiation detecting means, at least three image signals representing the radiation images are obtained in accordance with the at least three radiation detecting means. In such cases, as the two representative image signals, two arbitrary image signals selected from the at least three image signals may be employed.
Also, in cases where the stimulable phosphor sheet is employed as the radiation detecting means, a two-surface read-out technique may be employed, wherein the stimulating rays are irradiated to opposite surfaces of the stimulable phosphor sheet or only one surface of the stimulable phosphor sheet, and light emitted from one surface of the stimulable phosphor sheet and light emitted from the other surface of the stimulable phosphor sheet are detected photoelectrically. The two-surface read-out technique described above is described in, for example, U.S. Pat. No. 4,346,295. In cases where the two-surface read-out technique described above is employed, two image signals are obtained from one stimulable phosphor sheet. In the second energy subtraction processing method in accordance with the present invention, the two image signals having been obtained from one stimulable phosphor sheet with the two-surface read-out technique described above are also processed as the image signals representing two independent radiation images. In cases where the two-surface read-out technique described above is employed, a larger number of image signals than that of the radiation detecting means are obtained. In such cases, a mean signal value of the two image signals having been obtained from one stimulable phosphor sheet may be calculated and taken as one of the two representative image signals, which are representative of the plurality of the image signals.
The term xe2x80x9cradiation dose at the time of formation of a radiation imagexe2x80x9d as used herein means the dose of radiation, which carries the image information of the object and impinges upon the radiation detecting means when the image of the object is formed on the radiation detecting means. The radiation dose is capable of being obtained by directly detecting the radiation impinging upon the radiation detecting means. However, it is not always possible to detect the radiation dose with respect to each of the pixels in the radiation image. However, when the dose of radiation impinging upon the radiation detecting means is large, the image signal obtained from the radiation detecting means takes a large signal value. Thus the signal value of the image signal has the correspondence relationship with the radiation dose. Therefore, in the second energy subtraction processing method in accordance with the present invention, as in the first energy subtraction processing method in accordance with the present invention, the image signal, which has been obtained from the radiation detecting means (and has not yet been subjected to image processing), should preferably be regarded as the radiation dose and utilized for the setting of the mean attenuation coefficient.
The mean attenuation coefficient is set for each of specific structures contained in the object. Therefore, with respect to one radiation image, the same number of the mean attenuation coefficients as that of the kinds of the specific structures contained in the object are set. For example, in cases where the specific structures are the soft tissue and the bone of the human body, one mean attenuation coefficient of the soft tissue and one mean attenuation coefficient of the bone are obtained with respect to one radiation image.
As will be described later, in cases where the difference between the logarithmic values of the radiation doses at the time of the formation of two radiation images is calculated, or the logarithmic value of the ratio between the radiation doses at the time of the formation of the two radiation images is calculated, a certain relationship is obtained between the difference between the logarithmic values of the radiation doses, or the logarithmic value of the ratio between the radiation doses, and the mean attenuation coefficient of a certain substance with respect to each of the two radiation images, which are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses. Also, in cases where at least three radiation images are formed, the mean attenuation coefficient of a certain substance with respect to one of the at least three radiation images has a certain relationship with the difference between the logarithmic values of the radiation doses with respect to the other two radiation images, or with the logarithmic value of the ratio between the radiation doses with respect to the other two radiation images. Therefore, the mean attenuation coefficient of the certain substance with respect to the one radiation image is capable of being calculated in accordance with the difference between the logarithmic values of the radiation doses with respect to the other two radiation images, or in accordance with the logarithmic value of the ratio between the radiation doses with respect to the other two radiation images. Accordingly, for each of the combinations of the two radiation images, which two radiation images are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses and are selected from the plurality of the radiation images, the mean attenuation coefficients with respect to all of the radiation images are capable of being set.
For example, in cases where the specific structures are the soft tissue and the bone of the human body, and three radiation images are to be processed, with respect to each of the specific structures, there are three kinds of the combinations of the two radiation images, which two radiation images are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses. Therefore, for each of the three kinds of the combinations, the mean attenuation coefficients with respect to all of the radiation images (i.e., with respect to the three radiation images) are set. Also, for one radiation image, the mean attenuation coefficient is set for each of the soft tissue and the bone. Accordingly, in such cases, 18 kinds [=the number of the combinations (3)xc3x97the number of the radiation images (3)xc3x97the number of the specific structures (2)] of the mean attenuation coefficients are set.
The mean value of the mean attenuation coefficients, which have been set with respect to the identical radiation image among all of the radiation images and for all of the combinations of the two radiation images selected from the plurality of the radiation images, is calculated with respect to each of the radiation images and with respect to each of the specific structures contained in the object. For example, in cases where the number of the radiation images is three, as the mean value of the mean attenuation coefficients, which have been set with respect to the identical radiation image, three mean values are calculated for the three radiation images and with respect to each of the specific structures contained in the object. Therefore, in cases where three radiation images are to be processed, and the specific structures are the soft tissue and the bone of the human body, six kinds (=3xc3x972) of the mean values are obtained.
The mean value described above may be a simple arithmetic mean value. However, it is considered that, due to adverse effects of a noise contained in the radiation, scattered radiation, and the like, the mean attenuation coefficient deviates from a true value, i.e. the value which will be obtained in cases where the noise, the scattered radiation, and the like, are not contained in the radiation. Also, it is considered that the deviation of the mean attenuation coefficient occurs in the normal distribution. Therefore, the mean value of the mean attenuation coefficients described above should preferably be calculated with a weighted mean calculating process, in which the mean attenuation coefficients are weighted in accordance with standard deviations of the mean attenuation coefficients.
In the second energy subtraction processing method in accordance with the present invention, the difference signal is obtained from the two representative image signals, which are representative of the plurality of the image signals representing the plurality of the radiation images. As described above, in cases where the image recording operation is performed by utilizing at least three radiation detecting means, at least three image signals representing the radiation images are obtained in accordance with the at least three radiation detecting means. In such cases, two arbitrary image signals are selected as the two representative image signals from the at least three image signals and subjected to the energy subtraction processing. In such cases, as the representative values of the mean values, the mean values, which have been calculated with respect to the radiation images represented by the two representative image signals having been selected, are employed.
Also, in cases where the stimulable phosphor sheet is employed as the radiation detecting means, and the two-surface read-out technique described above are employed, the mean signal value of the two image signals having been obtained from one stimulable phosphor sheet may be calculated and taken as one of the two representative image signals, which are representative of the plurality of the image signals. In such cases, as one of the representative values of the mean values, the value obtained by averaging the mean values having been calculated with respect to the radiation images represented by the two image signals, from which the mean signal value has been calculated, may be employed.
As described above, in cases where the radiation doses with respect to two radiation images are represented by I1 and I2, the relationship represented by the formula ln(I1)xe2x88x92ln(I2)=ln(I1/I2) is obtained. Therefore, the xe2x80x9cdifference between the logarithmic values of the radiation doses with respect to the corresponding pixels in the radiation imagesxe2x80x9d and the xe2x80x9clogarithmic value of the ratio between the radiation doses with respect to the corresponding pixels in the radiation imagesxe2x80x9d take an identical value.
As for the relationship between the mean attenuation coefficient and the difference between the logarithmic values of the radiation doses with respect to the corresponding pixels in the radiation images, or the relationship between the predetermined weight factor and the logarithmic value of the ratio between the radiation doses with respect to the corresponding pixels in the radiation images, a table representing the relationship or a functional expression representing the relationship may be utilized. In cases where the table representing the relationship described above is utilized, reference may be made to the table, and the mean attenuation coefficient may thereby be set. In cases where the functional expression representing the relationship is utilized, operation processing may be performed with the functional expression, and the mean attenuation coefficient may thereby be set.
The relationship between the mean attenuation coefficient and the difference between the logarithmic values of the radiation doses describe above, or the relationship between the mean attenuation coefficient and the logarithmic value of the ratio between the radiation doses described above, varies for different image recording conditions employed in the image recording operation, such as the voltage of a radiation source, the kind of the radiation source, and the sensitivity of the radiation detecting means. Therefore, a plurality of tables or functions in accordance with various different image recording conditions should preferably be prepared previously, and a table or a function should preferably be selected in accordance with the image recording conditions. Also, the mean attenuation coefficient should preferably be set by the utilization of the thus selected table or the thus selected function.
As described above, in the second energy subtraction processing method in accordance with the present invention, the mean value of the mean attenuation coefficients should preferably be calculated with a weighted mean calculating process, in which the mean attenuation coefficients are weighted in accordance with standard deviations of the mean attenuation coefficients.
In such cases, the weighting of each of the mean attenuation coefficients having been set in accordance with a radiation image, which contains more of scattered radiation than the other radiation images among the plurality of the radiation images, should preferably be set to be lighter than the weighting of the mean attenuation coefficient having been set in accordance with the other radiation images.
By way of example, in cases where the plurality of the radiation images are formed with the one-shot energy subtraction processing technique, the radiation image, which has been obtained with the radiation detecting means located at a position close to the object, contains more of the scattered radiation than the radiation images, which have been obtained with the radiation detecting means located at positions remote from the object. Therefore, in such cases, the term xe2x80x9cradiation image containing more of scattered radiation than the other radiation imagesxe2x80x9d as used herein means the radiation image, which has been obtained with the radiation detecting means located at the position closest to the object.
In cases where the thickness of the object is large, much of scattered radiation occurs. Also, in cases where the thickness of the object is large, the dose of the radiation impinging upon the radiation detecting means becomes small. Therefore, in the second energy subtraction processing method in accordance with the present invention, with respect to the radiation image, which contains more of the scattered radiation than the other radiation images, the weighting of each of the mean attenuation coefficients having been set in accordance with the radiation image should preferably be set to be light in cases where the radiation dose is small.
The present invention also provides a second energy subtraction processing apparatus, comprising:
i) means for obtaining a plurality of image signals, each of which represents one of a plurality of, at least three, radiation images of a single object and is made up of a series of image signal components, the plurality of the radiation images having been formed respectively with a plurality of kinds of radiation having different energy distributions and carrying image information of the object, different images of at least part of the object being embedded in the plurality of the radiation images, and
ii) means for weighting each of two representative image signals, which are representative of the plurality of the image signals, with a predetermined weight factor, and performing a subtraction process on the image signal components of the weighted image signals, which image signal components represent corresponding pixels in the two radiation images represented by the two representative image signals, in order to obtain a difference signal representing an image of a specific structure of the object,
wherein the, improvement comprises the provision of:
a) mean attenuation coefficient setting means for setting a mean attenuation coefficient with respect to each of all of the radiation images, with respect to each of pixels in each of the radiation images, and in accordance with a difference between logarithmic values of radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images, or in accordance with a logarithmic value of a ratio between the radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images, the setting of the mean attenuation coefficient being performed for each of combinations of two radiation images, which two radiation images are associated with a calculation of the difference between the logarithmic values of the radiation doses or a calculation of the logarithmic value of the ratio between the radiation doses and are selected from the plurality of the radiation images,
b) mean value calculating means for calculating a mean value of the mean attenuation coefficients, which have thus been set with respect to an identical radiation image among all of the radiation images and for all of the combinations of the two radiation images selected from the plurality of the radiation images, the mean value of the mean attenuation coefficients being calculated with respect to each of pixels in the identical radiation image, a plurality of mean values being obtained with respect to all of the radiation image, and
c) setting means for setting representative values of the mean values, which representative values correspond to the radiation images represented by the two representative image signals, as the predetermined weight factors for the two representative image signals.
The second energy subtraction processing apparatus in accordance with the present invention should preferably be modified such that the apparatus further comprises storage means for storing information representing a table or a function, which represents a relationship between the mean attenuation coefficient and the difference between the logarithmic values of the radiation doses with respect to the corresponding pixels in the radiation images, or which represents a relationship between the mean attenuation coefficient and the logarithmic value of the ratio between the radiation doses with respect to the corresponding pixels in the radiation images, the relationship having been determined previously, and
the mean attenuation coefficient setting means makes reference to the table or the function having been stored in the storage means and sets the mean attenuation coefficient.
In such cases, the second energy subtraction processing apparatus in accordance with the present invention should more preferably be modified such that the storage means stores a plurality of tables or functions, which represent the relationships having been set in accordance with various different image recording conditions at the time of formation of radiation images, and
the mean attenuation coefficient setting means accepts selection of a table or a function in accordance with image recording conditions :having been set at the time of the formation of the plurality of the radiation images, makes reference to the thus selected table or the thus selected function, and thereby sets the mean attenuation coefficient.
In the second energy subtraction processing apparatus in accordance with the present invention, the mean value calculating means should preferably be means for calculating the mean value of the mean attenuation coefficients with a weighted mean calculating process, in which the mean attenuation coefficients are weighted in accordance with standard deviations of the mean attenuation coefficients.
In such cases, the mean value calculating means should preferably set the weighting of each of the mean attenuation coefficients having been set in accordance with a radiation image, which contains more of scattered radiation than the other radiation images among the plurality of the radiation images, to be lighter than the weighting of the mean attenuation coefficient having been set in accordance with the other radiation images.
Also, in such cases, in the second energy subtraction processing apparatus in accordance with the present invention, the mean value calculating means should preferably operate such that, with respect to the radiation image, which contains more of the scattered radiation than the other radiation images, the weighting of each of the mean attenuation coefficients having been set in accordance with the radiation image is set to be light in cases where the radiation dose is small.
The present invention further provides a second recording medium, on which a program for causing a computer to execute the second energy subtraction processing method in accordance with the present invention has been recorded and from which the computer is capable of reading the program.
The first energy subtraction processing method and the first energy subtraction processing apparatus in accordance with the present invention are based on the findings that the certain relationship is obtained between the difference between the logarithmic values of the radiation doses with respect to the radiation images, or the logarithmic value of the ratio between the radiation doses with respect to the radiation images, and the mean attenuation coefficient, i.e. the weight factor. With the first energy subtraction processing method and the first energy subtraction processing apparatus in accordance with the present invention, on the basis of the findings described above, the predetermined weight factor is set with respect to each of the pixels in each of the radiation images and in accordance with the difference between the logarithmic values of the radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images, or in accordance with the logarithmic value of the ratio between the radiation doses with respect to the corresponding pixels in the radiation images at the time of the formation of the plurality of the radiation images. The radiation dose with respect to each of the pixels in each of the radiation images varies in accordance with the thickness of the specific structure contained in the object. Therefore, the predetermined weight factor, which has been set with respect to each of the pixels in each of the radiation images and in accordance with the difference between the logarithmic values of the radiation doses with respect to the corresponding pixels in the radiation images, or in accordance with the logarithmic value of the ratio between the radiation doses with respect to the corresponding pixels in the radiation images, takes a value in accordance with the thickness of the specific structure. Accordingly, in cases where the image signals are multiplied by the predetermined weight factors, which have thus been set, and the thus weighted image signals are subtracted from each other, regardless of the thickness of the structure contained in the object, a pattern of an unnecessary structure is capable of being removed approximately perfectly. As a result, the difference signal representing an image, in which only a pattern of the specific structure is illustrated accurately, is capable of being obtained.
Also, with the first energy subtraction processing method and the first energy subtraction processing apparatus in accordance with the present invention, the table or the function, which represents the relationship between the predetermined weight factor and the difference between the logarithmic values of the radiation doses, or which represents the relationship between the predetermined weight factor and the logarithmic value of the ratio between the radiation doses, may be determined previously. In such cases, reference may be made to the table or the function, and the predetermined weight factor is capable of being set easily. Therefore, the calculation of the difference signal is capable of being made efficiently.
Further, with the first energy subtraction processing method and the first energy subtraction processing apparatus in accordance with the present invention, the plurality of the tables or the functions, which represent the relationships having been set in accordance with various different image recording conditions, may be prepared previously. Also, a table or a function may be selected in accordance with the image recording conditions having been set at the time of the formation of the plurality of the radiation images. Reference may be made to the thus selected table or the thus selected function, and the predetermined weight factor may thereby be set. In such cases, only the pattern of the specific structure is capable of being extracted accurately regardless of the image recording conditions.
The second energy subtraction processing method and the second energy subtraction processing apparatus in accordance with the present invention are based on the findings that the certain relationship is obtained between the difference between the logarithmic values of the radiation doses with respect to the radiation images, or the logarithmic value of the ratio between the radiation doses with respect to the radiation images, and the mean attenuation coefficient of a certain substance with respect to each of the two radiation images, which are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses. With the second energy subtraction processing method and the second energy subtraction processing apparatus in accordance with the present invention, on the basis of the findings described above, in accordance with the difference between the logarithmic values of the radiation doses with respect to the radiation images, or in accordance with the logarithmic value of the ratio between the radiation doses with respect to the radiation images, the mean attenuation coefficient is calculated for each of the combinations of the two radiation images, which two radiation images are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses. Specifically, the mean attenuation coefficient is set with respect to each of all of the radiation images, and the setting of the mean attenuation coefficient is performed for each of the combinations of the two radiation images, which two radiation images are associated with the calculation of the difference between the logarithmic values of the radiation doses or the calculation of the logarithmic value of the ratio between the radiation doses. Also, a calculation is made to find the mean value of the mean attenuation coefficients, which have thus been set with respect to an identical radiation image among all of the radiation images and for all of the combinations of the two radiation images selected from the plurality of the radiation images. The mean value of the mean attenuation coefficients is calculated with respect to each of the pixels in the identical radiation image. A plurality of the mean values are thus obtained with respect to all of the radiation image. Further, the representative values of the mean values, which representative values correspond to the radiation images represented by the two representative image signals to be subjected to the energy subtraction processing, are determined. The thus determined representative values are set as the predetermined weight factors for the two representative image signals, and the energy subtraction processing is performed by the utilization of the predetermined weight factors.
Therefore, in cases where the image signals are multiplied by the predetermined weight factors, which have thus been set, and the thus weighted image signals are subtracted from each other, regardless of the thickness of the structure contained in the object, a pattern of an unnecessary structure is capable of being removed approximately perfectly. As a result, the difference signal representing an image, in which only a pattern of the specific structure is illustrated accurately, is capable of being obtained.
As described above, in the strict sense, the mean attenuation coefficient deviates from the true value due to adverse effects of the noise and the scattered radiation. However, with the second energy subtraction processing method and the second energy subtraction processing apparatus in accordance with the present invention, the plurality of the mean attenuation coefficients are calculated with respect to each of the radiation images, and the mean value of the mean attenuation coefficients is calculated with respect to each radiation image. Also, the representative values of the thus calculated mean values are set as the predetermined weight factors in the energy subtraction processing. Therefore, the mean attenuation coefficients comparatively close to the true values are capable of being utilized in the energy subtraction processing. Accordingly, the pattern of the specific structure is capable of being extracted accurately.
Also, with the second energy subtraction processing method and the second energy subtraction processing apparatus in accordance with the present invention, the table or the function, which represents the relationship between the mean attenuation coefficient and the difference between the logarithmic values of the radiation doses, or which represents the relationship between the mean attenuation coefficient and the logarithmic value of the ratio between the radiation doses, may be determined previously. In such cases, reference may be made to the table or the function, and the mean attenuation coefficient is capable of being set easily. Therefore, the calculation of the difference signal is capable of being made efficiently.
Further, with the second energy subtraction processing method and the second energy subtraction processing apparatus in accordance with the present invention, the plurality of the tables or the functions, which represent the relationships having been set in accordance with various different image recording conditions, may be prepared previously. Also, a table or a function may be selected in accordance with the image recording conditions having been set at the time of the formation of the plurality of the radiation images. Reference may be made to the thus selected table or the thus selected function, and the mean attenuation coefficient may thereby be set. In such cases, only the pattern of the specific structure is capable of being extracted accurately regardless of the image recording conditions.
Furthermore, it is considered that the deviation of the mean attenuation coefficient from the true value occurs in the normal distribution. Therefore, with the second energy subtraction processing method and the second energy subtraction processing apparatus in accordance with the present invention, the mean value of the mean attenuation coefficients described above may be calculated with the weighted mean calculating process, in which the mean attenuation coefficients are weighted in accordance with the standard deviations of the mean attenuation coefficients. In such cases, the predetermined weight factors are capable of being set accurately.
Also, deviations of the mean attenuation coefficients having been set in accordance with a radiation image, which contains much of the scattered radiation, from the true values are large due to the adverse effects of the scattered radiation. Therefore, with the second energy subtraction processing method and the second energy subtraction processing apparatus in accordance with the present invention, the weighting of each of the mean attenuation coefficients, which have been set in accordance with a radiation image containing more of scattered radiation than the other radiation images among the plurality of the radiation images, in accordance with the standard deviations of the mean attenuation coefficients may be set to be lighter than the weighting of the mean attenuation coefficient having been set in accordance with the other radiation images. In such cases, the effects of the mean attenuation coefficients, which deviate largely from the true values due to the scattered radiation, are capable of being suppressed. Therefore, the predetermined weight factors are capable of being set more accurately.
Further, in cases where the thickness of the object is large, the radiation dose becomes small, and the amount of the scattered radiation becomes large. Therefore, in the second energy subtraction processing method in accordance with the present invention, with respect to the radiation image, which contains more of the scattered radiation than the other radiation images, the weighting of each of the mean attenuation coefficients, which have been set in accordance with the radiation image, in accordance with the standard deviations of the mean attenuation coefficients may be set to be light in cases where the radiation dose is small. In such cases, the adverse effects of the scattered radiation are capable of being minimized.